Multimode optical fiber is a ubiquitous medium that is installed in a variety of applications, such as with universities, schools, hospitals, businesses and factories. In local and campus area networks, multimode fiber (MMF) has often been favored over single-mode fiber (SMF) because of the low cost of fiber installation and maintenance, and because of the much lower cost of transceiver components for MMF. In this context, “mode” generally refers to the characteristic of the propagation of light (e.g., through a waveguide) that can be designated by a radiation pattern in a plane transverse to the direction of travel. “Single-mode fiber (SMF)” thus has been used to generally refer to a fiber that facilitates light propagation that is designated by a single light characteristic type (i.e., a single radiation pattern). “Multi-mode fiber (MMF)” thus has been used to generally refer to a fiber that facilitates light propagation that is designated by two or more light characteristic types (i.e., two or more radiation patterns).
From a practical standpoint, MMF has generally offered lower capacity than SMF. Transmission rates in MMF are limited by the propagation of multiple transverse modes at different group velocities; this is typically referred to as modal dispersion, e.g., wherein a signal is spread in time due to different propagation velocities for different modes. SMF is typically free of this type of modal dispersion. Hence, in recent decades, research on SMF systems has far outstripped work on MMF systems. SMF systems can transport terabits per second over thousands of kilometers. However, MMF systems have typically been limited to a bit rate-distance product well below 10 Gb/s-km.
In some aspects, wireless channel communications are analogous to MMF communications. Multipath fading in wireless systems was traditionally viewed as a strictly deleterious phenomenon. Various techniques have been devised to overcome fading in single-input, single-output (SISO) links, including diversity and equalization, as well as multicarrier and spread spectrum modulation. In recent years, it has been realized that multipath fading actually creates additional spatial dimensions that can be exploited by multi-input, multi-output (MIMO) techniques to dramatically enhance wireless transmission capacity. The plurality of modes in MMF has traditionally been viewed as a strictly negative, bandwidth-limiting effect, and various techniques have been proposed to counter modal dispersion.
Various approaches for eliminating modal dispersion in SISO MMF links have been proposed. For example, multimode fibers with substantially low modal dispersion have been developed. Wavelength-division multiplexing (WDM) can be used to increase the aggregate bit rate (but is relatively high in cost). Various other techniques, including controlled launch, electrical equalization or subcarrier modulation can provide relatively limited increase in bit rate-distance product associated with multimode fibers.
Among less conventional approaches, a segmented photodetector can be used to perform spatially resolved intensity detection. The photocurrents from the different segments can be processed using diversity combining and electrical equalization to mitigate the effect of modal dispersion. Another approach involves the use of diffractive optical elements to selectively excite one fiber mode in an attempt to reduce modal dispersion. With these approaches, fixed spatial filtering is used to launch into one “mode,” which is more precisely described as an eigenmode of an idealized round, straight fiber. In real fibers, random fabrication errors and bends lead to coupling between these ideal eigenmodes over distances of centimeters to meters. Hence, even if one launches into a single ideal eigenmode, substantial modal dispersion still occurs over transmission distances of practical interest, which are tens to thousands of meters. Furthermore, in the presence of modal coupling, slow changes in the fiber temperature and stress make modal dispersion time-varying, on a time scale typically of the order of seconds. It is interesting to note that while modern graded-index fibers have far less modal dispersion than older step-index fibers, the velocity matching in graded-index fibers actually enhances mode coupling, making it even more difficult to control modal dispersion by launching into one ideal eigenmode.
Over the years, several groups have proposed various approaches to MIMO transmission in MMF. For instance, it has been suggested to exploit multiple spatial degrees of freedom using angle multiplexing, i.e., by launching different information streams at different angles. While this approach is tantalizing and has been proposed by several groups over the past two decades, it has failed because signals launched at different angles become cross-coupled after propagating a few meters in step-index or graded-index fibers.